Up until now, the image forming performance of an image forming apparatus is evaluated in such a manner as to compare differences between the colors of an image output from the image forming apparatus such as a printer and the colors of original image information. Generally, the image forming performance is evaluated according to the following method. In other words, a chart image for color evaluation is first output using prepared image information and then read by a scanner to obtain output image information. After that, in regard to the colors of respective parts in the chart image, differences between the colors expressed by the original image information and the colors expressed by the output image information are calculated to thereby evaluate the image forming performance.
In recent years, however, demand for outputting photographic images or the like has been increasing. Therefore, according to such an evaluation method, the above-described chart image for color evaluation has to be output onto an expensive gloss photo paper, which results in an increase in cost. Further, in the case of outputting the same images in large amounts, the likelihood of obtaining a good result would be high by using the colors of an image to be actually output as objects to be inspected rather using than a chart image for color evaluation including only limited colors. Therefore, there have been demanded techniques for evaluating image forming performance using an image freely output by the user instead of a chart image for color evaluation.
In order to evaluate image forming performance using an image freely output by a user, a technique for extracting an object region suitable for color evaluation from the entire region of the image is necessary. As such, a region extraction method disclosed in Patent Document 1 is known. According to this region extraction method, a small segment region including a target pixel is first extracted from the entire region of an original image based on image information, and then an entropy value indicating evenness (uniformity) in density between respective pixels in the small segment region is calculated based on the pixel values of the respective pixels in the small segment region. After repeatedly performing the processing of extracting small segment regions and calculating entropy values thereof while sequentially shifting target pixels, the small segment region in which density between the respective pixels is even is specified from the entire region of the image based on the entropy values of the respective small segment regions. The region in which the density between the respective pixels is even is suitable for evaluating an output color because it has less color variations. That is, with the application of the region extraction method disclosed in Patent Document 1, it is possible to extract an object region suitable for color evaluation from the entire region of an original image provided by the user.
However, even if an object region suitable for color evaluation can be extracted, the adjustment of image forming performance excellent in color reproducibility is not always made possible. Specifically, a general image forming apparatus has at least a characteristic in which the reproducibility of one color is degraded as it adjusts image forming performance so as to faithfully reproduce another color. Thus, even if an object region suitable for color evaluation is extracted by the application of the region extraction method disclosed in Patent Document 1, when image forming performance is adjusted to bring its output color in close to an original color based on the measurement result of an actual output color of the object region, the color reproducibility of other regions is greatly degraded. Accordingly, the color reproducibility of an image could be rather degraded as a whole.
Further, image forming performance cannot be accurately evaluated only by the extraction of region in which a difference in density is even. In order to accurately evaluate image forming performance, it is requested that the regions have a color tone close to a color material (color purity is high) and are adequately dispersed in the entire region of an image (spatial dispersion degree is high), besides the evenness of a difference in density (evenness degree is high). Specifically, in the image forming apparatus that outputs color images, at least three different color materials, such as Y (yellow), M (magenta), and C (cyan), are used as the color materials of ink and toner. The image forming apparatus reproduces various color tones by appropriately mixing such monochromatic color materials together on a paper or adjusting an area ratio of single-color dots composed of only the respective color materials. In order to accurately evaluate color reproducibility in such a configuration, it is necessary to select, as object regions, regions having color tones close to the color materials in such a manner that single-color regions close in color tone to Y, M, and C are selected as the regions to be detected. Further, the image forming apparatus is likely to show different color reproducibility depending on the position of an image; the color reproducibility is different between the upper side and the lower side of a paper even with the same color. Thus, it is insufficient to specify only one region from the entire region of an image as an object region for the respective single colors such as Y, M, and C, but is necessary to specify plural regions appropriately dispersed in the image as object regions. Accordingly, with respect to the respective single colors, it is necessary to specify plural combinations of segment regions showing a relatively high evenness degree and color purity in which the spatial dispersion degrees of the segment regions are relatively high from the entire region of the image.
In order to specify combinations of such segment regions, the present inventor has conceived the following method. In other words, the processing of extracting segment regions having a predetermined size from the entire region of an image and then calculating the evenness degree and the color purity of the segment regions is repeatedly performed until the entire image is covered. Next, all possible combinations established when a predetermined number of the segment regions are selected from all segment regions and combined with each other are specified. Then, the evenness degree, the color purity, and the linear sum of the spatial dispersion degree of the respective segment regions are calculated for the respective combinations and regarded as index values. Here, one of the combinations showing the largest index value is specified as an object region for inspecting an output color.
However, it turns out that this method is not practical because it requires an enormous processing time for calculating the above-described linear sum for the possible combinations established when the predetermined number of the segment regions are selected from all the segment regions and combined with each other.    Patent Document 1: JP-B-3860540